A delicate balance of cell renewal, differentiation and cell death within the intestinal epithelium is crucial to maintain the gastrointestinal tissue structure and function which form the basis for normal physiology of the gut. Intestinal epithelium undergoes a programmed self-renewal throughout adult life and a rapid regeneration after injury due to the presence of multipotent epithelial stem and progenitor cells. Various signaling pathways have been implicated to play a role in the control of stem cell activity, proliferation, lineage commitment, terminal differentiation and cell survival during normal development and homeostasis of the gastrointestinal epithelium. Deregulation of these signaling pathways can lead to intestinal neoplasia. In this proposal, we focus on the elucidation of a novel protein kinase signaling pathway that may play an important role in regulating the development and regeneration of the intestinal epithelium. Intestinal cell kinase (ICK) is a highly conserved serine/threonine protein kinase. In the small intestine, the ICK mRNA localizes specifically to the undifferentiated intestinal epithelium in the lower crypt compartment where stem cells reside suggesting that ICK may play a role in epithelial replication, lineage specification and cell fate determination in crypt epithelium. Our preliminary data shows [sic] that ICK expression in crypt epithelium is significantly induced during crypt epithelium regeneration when stem/progenitor cells actively replicate. Our in vitro knockdown studies in intestinal epithelial cell lines suggest that ICK promotes intestinal epithelial proliferation and G1 cell cycle progression, possibly by selectively regulating protein translation of key cell cycle regulators through the mammalian target of rapamycin (mTOR) pathway. Taken together these data support our current working hypothesis that ICK mediated signaling regulates proliferation and differentiation of epithelial stem cells and their progenitors in the intestinal epithelium during normal intestinal ontogeny and/or during the crypt epithelium regeneration in response to mucosal injury. To test this hypothesis, we propose three specific aims. Aim 1: Determine whether ICK mediated signaling regulates epithelial proliferation and differentiation in vivo during normal development and homeostatic maintenance of the intestinal epithelium; Aim 2: Determine whether ICK mediated signaling is important for intestinal epithelial repair and regeneration in response to mucosal injury; Aim 3: Determine whether ICK mediated signaling regulates intestinal epithelial cell proliferation and differentiation by modulating protein translation of cell cycle regulators through the mTOR signaling pathway.